Conventional switch devices operate to control the path on which a signal travels. One example of a transistor-based switch is comprised of a plurality of field effect transistors (FETs). FET switches are generally known to be used in connection with high frequency signal transmission, for example, radio frequency (RF).
In general, an n-channel FET switch is in an ON state (very low impedance) allowing any signal to pass from the source to the drain of the FET until a control voltage applied to the gate of the FET falls below a predetermined amount. When the control voltage is removed, the FET switches to an OFF state (very high impedance) and prevents any signal passing from the source to the drain of the FET. The control voltage is selected such that the magnitude of the gate-to-source voltage of the FET, Vgs, rises above the magnitude of a threshold “pinchoff” voltage Vp. The advantage of a FET switch is that the control voltage applied to the gate of the FET draws very little current, consuming little power in performing the switching function. Switches may be combined with shunts, for example, in applications where a switch is used between two or more signal ports so as to increase isolation between the ports.
For example, FIG. 1 shows a series-shunt arm of a prior art switch. The arm includes a series path between two signal ports, as provided by a series FET 16. A shunt FET 24 provides a shunt path to enhance the isolation between the two signal ports when the arm is in an OFF state. In the OFF state, the channel of the series FET 16 is biased such that Vgs falls below Vp, creating a high impedance between the two signal ports dominated by the junction capacitances of the series FET. However, the Vgs of the shunt FET 24 is biased by V′ to be above |Vp|, creating a low impedance path to ground. Although the series FET 16 alone provides a great deal of isolation between the two signal ports, the low impedance path to ground, as provided by the shunt FET, enhances the isolation. Since for the OFF state the bias voltage V for the series FET is below (VRF1−Vp) and the bias voltage V′ for the shunt FET is above |Vp|, then the equivalent logic states that control these arms are inverted from one another. Therefore, opposite logic states must be available to switch the path between the two signal ports from least attenuation to maximum isolation.
In the ON state for the series-shunt arm, the channel of the series FET 16 is biased above Vp creating a low impedance between the two signal ports. However, the channel of the shunt FET 24 is biased below Vp creating a high impedance path to ground. With such equivalent logic states or biases applied to the switch, minimum attenuation is achieved by decreasing the loss between the signal ports and minimizing the coupling of the signal to ground through the shunt path. Similar to the OFF state, the series and shunt FETs require opposite logic states.
FIG. 2 discloses a prior art switch similar to FIG. 1, but having a multistage configuration. The switch of FIG. 2 also requires opposite logic states for the proper biasing of the series FETs and shunt FET.
Typical control logic for such prior art switches includes various active and passive components.
A feedforward capacitor with a low impedance can be used to improve the harmonic rejection of an FET. By improving the harmonic rejection, signal distortions and noise interferences can be reduced or eliminated, and the performance of the FET structures can be improved greatly. Feedforward capacitors are often employed in designs seeking high isolation and power handling. The feedforward capacitor may be coupled across the gate of an FET and a signal port.